In many imaging applications, an image sensor needs to capture a high dynamic range (DR) of light intensities in an image. Classic image sensors have S/N ratio's (SNR) in the order of 1000:1 to 10000:1. When such sensor has a linear response, it can capture at most a dynamic range of a factor 10000:1 in light intensities. In order to increase the dynamic range of image sensors, many techniques are known in the art, which may use, for example, a non-linear response curve, e.g. multiple piece-wise linear slopes, logarithmic responses or log-lin responses, non-destructive readout, charge coupled devices (CCD) with two wells, overflow MOSFET capacitors, a combination of multiple shorter integration periods or smart reset pixels.
In European patent application EP 2346079, Wang et al. disclose a pixel structure comprising a photo-sensitive element, a first transfer gate connected between the photo-sensitive element and a first charge conversion element. A second transfer gate is connected between the photo-sensitive element and a second charge conversion element.
In U.S. Pat. No. 7,821,560, Sugawa et al. disclose a pixel structure comprising a photodiode, a transfer transistor coupled to the photodiode, and a plurality of capacitors for storing photocharges overflowing from the photodiode through the transfer transistor in storage operation. This “overflowing” implies a “storage gate” MOSFET between subsequent capacitance elements. In all drawings Sugawa shows capacitors separated by storage gates.
FIG. 1 illustrates a pixel element in accordance with a prior art technique, also referred to in Sugawa et al., in which a transistor switch T is provided between a photodiode PD and a capacitance C. The switch T is turned on in a first exposure period to store photosignal charges in both the photodiode PD and the capacitance C and is turned off in a second exposure period to store photocharges in the photodiode PD in addition to the already stored charge in capacitance C, such that a wide dynamic range may be obtained.